Failing Tethers Destabilize Neutrophil Rolling and Form Elongated Neutrophil-Derived Structures (ENDS) in the Blood Circulation

Neutrophils are the most abundant immune cells in human blood and are crucial for host defense against bacterial and fungal infections. Neutrophils are made mainly in the bone marrow from where they travel to different tissues with the blood circulation. Neutrophils are recruited from the blood stream into the tissue through a cascade of events that entail neutrophil rolling and arrest on the endothelium and migration through the vessel wall. Neutrophils roll at a velocity about 1000-fold less compared to the blood flow velocity, which enables the neutrophils to screen the vessel wall for inflammatory mediators and facilitates arrest. The importance of neutrophil rolling is demonstrated by the increased risk of severe bacterial infection in patients harboring genetic deficiency in selectin mediated neutrophil interaction with the endothelium, that is crucial for rolling. Neutrophil rolling is further stabilized by the formation of long thin membrane tethers from the endothelial bound microvilli of the rolling neutrophils. We found that tethers can fail and detach from the neutrophil. This will result in unstable neutrophil rolling and often detachment of the rolling neutrophil from the vessel wall. The separated tethers will form sliding, adhering or circulating elongated neutrophil-derived structures (ENDS) in the vessel lumen. We found ENDS to be devoid of mitochondria, endoplasmic reticulum, or DNA, but enriched for S100A8, S100A9. In in vitro experiments within hours of formation, ENDS round up and release S100A8–S100A9 complex, a damage-associated molecular pattern protein that is a known biomarker of neutrophilic inflammation. We detected 10–100-fold elevated ENDS count in blood plasma of septic patients compared to healthy donors. Unlike neutrophil-derived extracellular vesicles, ENDS do not stain for CD9, CD63, and CD81. Thus, ENDS are a new class of bloodborne microparticles with a formation mechanism linked to neutrophil rolling on the vessel wall.

Learning objective: 

1. Explain neutrophils and neutrophil rolling.

2. Explain the consequences of tether detachment from rolling neutrophils.

3. Explain how ENDS compare to other neutrophil-derived extracellular particles.